Caption: John Dalton (1766-1844), British chemist, physicist and meteorologist. Dalton is most famous for his theory that matter is made of small indivisible units called atoms. This idea had also been proposed by the Ancient Greek philosopher Democritus. However, it was not until the 18th and 19th century that Dalton and his atomic theory explained chemical changes. Dalton published his theory in A New System of Chemical Philosophy (1808). Other work by Dalton included the Law of Partial Pressures for gases (1801) and an important paper on colour blindness (1794).